M. Sc II Semester (Biotechnology) Cells, Molecules and Developmental Biology Unit IV Types of Gametogenesis: Spermatogenesis and Oogenesis Gametogenesis is the process of formation and differentiation of haploid gametes (sperms and ova) from the diploid primary germ cells, gametogonia (spermatogonia and oogonia) present in primary sex organs called gonads (testes in male and ovaries in female respectively). Gametogenesis is of two types: I. Spermatogenesis and II. Oogenesis. I. Spermatogenesis Definition: It is the formation of haploid, microscopic and functional male gametes, spermatozoa from the diploid reproductive cells, spermatogonia, present in the testes of male organism. Period: In the seasonally breeding animals, the testes undergo testicular cycle in which the testes and their spermatogenic tissue become functional only in the specific breeding season. So in some seasonally breeding mammals like bat, otter and llama, testes enlarge, become functional and descend into the scrotum in the breeding season as become heavier due to accumulation of sperms, while become reduced, non-functional and ascend into the abdomen in other seasons. But in human male, lion, bull, horse etc., the testes lie permanently in the scrotum and spermatogenesis occurs throughout the year. In human male, testes descend into the respective scrotal sacs during seventh month of development under the stimulation of FSH of adenohypophysis. But in some mammals e.g. elephant, echidna, dolphin, whale, seal etc., testes lie permanently in the abdomen (intra-abdominal) mainly due to presence of blubber (thick fatty layer beneath the skin). Spermatogenesis is a continuous process and is completed in about 74 days. Mechanism: Spermatogenesis is divided into two parts: A. Formation of Spermatid: It is divided into three phases: 1. Multiplicative or Mitotic phase: It involves the rapid mitotic division of diploid primary or primordial germ cells, called gonocytes, present in germinal epithelium of the seminiferous tubules of the testes. These cell are undifferentiated and have large and chromatin-rich nucleus. This forms large number of diploid and rounded sperm mother cells called spermatogonia (Gr. sperma = seed; gone = offspring). Each spermatogonial cell is about 12 pm in diameter and has a prominent nucleus. Some spermatogonia act as stem cells (called Type A spermatogonia) and go on dividing and adding new cells by repeated mitotic divisions, so forming spermatogenic lineage, but some spermatogonia move inward and enter growth phase (called Type B spermatogonia). 2. Growth phase: It is characterized by spermatocytogenesis in which a diploid spermatogonium increases in size (about twice) by the accumulation of nutritive materials (derived from germinal cells and not synthesized) in the cytoplasm and replication of DNA, and forms diploid primary spermatocyte. Nutritive materials are derived from germinal cells. During this, the primary spermatocyte prepares itself to enter meiosis. Growth phase of spermatogenesis is of much shorter duration than that of oogenesis. 3. Maturation or Meiotic phase: It is characterised by meiosis. The diploid primary spermatocyte undergoes meiosis-I (reductional or heterotypical division) and forms two haploid cells called secondary spermatocytes, each containing 23 chromosomes. It is immediately followed by meiosis-II (equational or homotypical division) in each secondary spermatocyte to form two haploid spermatids, each of which has 23 chromosomes. So each diploid spermatogonium produces 4 haploid spermatids. Different stages of spermatogenesis are interconnected by cytoplasmic strands till spermiogenesis when the maturing and interconnected gametes separate from each other. B. Spermiogenesis The transformation of a non-motile, rounded and haploid spermatid into a functional and motile spermatozoan is called spermiogenesis or spermioteliosis. The main aim is to increase the sperm motility by reducing weight and development of locomotory structure. It involves the following changes: 1. Nucleus becomes condensed, narrow and anteriorly pointed due to loss of materials like RNAs, nucleolus and most of acidic proteins. 2. A part of Golgi body of spermatid forms the acrosome, while the lost part of Golgi body is called Golgi rest. 3. Centrioles of spermatid form the neck of sperm. 4. Distal centriole gives rise to axoneme. 5. Mitochondria form a spiral ring behind the neck around the distal centriole and proximal part of axoneme. This is called nebenkern. 6. Most of cytoplasm is lost but some cytoplasm forms sheath of tail of sperm. The spermatids mature into spermatozoa in deep folds of the cytoplasm of the Sertoli cells (nurse cells) which also provide nourishment to them. Mature spermatozoa are released in the lumen of seminiferous tubules, called spermiation. The two testes of young adult form about 120 million sperms each day. Changes in spermatid to form sperm during spermiogenesis. Structure of spermatid Changes in the sperm 1. Nucleus Shrinks and elongated. 2. Golgi complex Changes to acrosome. 3. Distal centriole Forms axial filament of sperm tail. 4. Mitochondria Form mitochondrial spiral of sheath called nebenkem. 5. Cytoplasm Generally lost except a thin sheath called manchette. Control: In human male, spermatogenesis starts only at the age of puberty due to increased secretion of gonadotropin releasing hormone (GnRH) from the hypothalamus of brain. GnRH stimulates adenohypophysis to secrete two gonadotropins: FSH and ICSH. ICSH stimulates the Leydig’s cells of testis to secrete male sex hormones, called androgens, most important of which is testosterone. Testosterone stimulates the spermatogenesis especially spermiogenesis. FSH stimulates the Sertoli cells of testis to secrete certain factors which helps in the process of spermatogenesis. It is called physiological control. Types: In man and a large number of other animals having XY mechanism in male, there are two types of sperms: 50% Gynosperms having X-Chromosome and 50’X) Androsperms having Y- Chromosome. Significance: (a) Produces haploid sperms. (b) Crossing over may occur during meiosis-I, so producing variations. (c) Proves evolutionary relationship. II. Oogenesis: Definition: It involves the formation of haploid female gametes called ova, from the diploid egg mother cells, oogonia, of ovary of female organism. It involves 2 biological processes: Genetical programming and packaging. Period: Period of oogenesis is different in different animals. In human female, there are about 1,700 primary germ cells in the undifferentiated female gonad at one month of foetal development. These proliferate to form about 600,000 oogonia at two months of gestation period and by its 5th month, the ovaries contain over 7 million oogonia; however, many undergo atresia (degeneration of germ cells) before birth. At the time of birth, there are 2 million primary follicles, but 50% of these are atretic. Atresia continues and at the time of puberty each ovary contains only 60,000-80,000 primary follicles. Oogenesis is completed only after the onset of puberty and only one out of 500 is stimulated by FSH to mature. So oogenesis is a discontinuous and wasteful process. Mechanism: Like the spermatogenesis, oogenesis is formed of three phases: 1. Multiplicative phase: In this certain primary germ cells (larger in size and having large nuclei) of germinal epithelium of ovary undergo rapid mitotic divisions to form groups of diploid egg mother cells, oogonia. Each group is initially a chord and is called egg tube of pfluger which later forms a rounded mass, egg nest. 2. Growth phase: Growth phase of oogenesis is of very long duration than that of spermatogenesis e.g., only three days in Drosophila, 6-14 days in hen, 3 years in frog and many years (12-13 years) in human female. During growth phase, one oogonium of egg nest is transformed into diploid primary oocyte while other oogonia of the egg nest form a single-layered nutritive follicular epithelium around it. The structure so formed is called primary follicle. Later, each primary follicle gets surrounded by more layers of granulosal cells and changes into secondary follicle. Soon secondary follicle develops a fluid-filled antral cavity called antrum, and is called tertiary follicle. It further changes to form Graafian follicle. So not all the oogonia develop further. Growth phase involves: (a) Increase in size of oocyte (2000 times in frog; 43 times in mouse; 90,000 times in Drosophila; 200 times in hen and about 200 times in human female) by the formation and accumulation of yolk (vitellogenesis) by a special mitochondrial cloud lying close to nucleus and called yolk nucleus. (b) Nucleus becomes bloated with nucleoplasm and is called germinal vesicle. (c) A thin vitelline membrane is secreted around the oocyte. (d) Increase in number of mitochondria, amount of ER and Golgi body. (e) Formation of lampbrush chromosomes in fishes, amphibians, reptiles, birds, insects, etc. for rapid yolk synthesis. (f) Gene-amplification or redundancy of r-RNA genes for rapid synthesis of r-RNA. 3. Maturation phase: It is characterized by meiosis. In this, the diploid and fully grown primary oocyte undergoes meiosis-I (reductional division) to form two unequal haploid cells. The smaller cell is called first polar body (Polocyte) and has very small amount of cytoplasm. The larger cell is called secondary oocyte and has bulk of nutrient-rich cytoplasm. Both of these are haploids and each has 23 chromosomes. Secondary oocyte undergoes meiosis-II (equational